Presented at the 8th International Geosynthetics Conference. Yokohama, Japan.This paper describes experiments on the feasibility of using geotextiles to filter organics from runoff and combined sewer overflows. A horizontal flow permittivity test apparatus was used as a prototype to test 15cm x 20cm coupons. Non-woven, needle-punched products supported both depth (interior) and cake(surface) filtration modes. Crushed anthracite in the #100-#200 mesh range was used as an inert, replicable analogue of low density, irregularly shaped suspended particles. The AOS detected with this material differs from that measured with standard media, indicating different interaction with the geotextile pore structure.Drag forces maintained a cake on the upstream face of the filter even at low hydraulic gradients. The permittivity decreased as this coating thickened. When flow ceased, the organic cake sloughed off the upstream face of the coupon. Measurements made after several doses showed recovery of much of the original permittivity, with the loss in hydraulic capacity attributed to embedment of particles in the matrix

Cheng, S.C

Marino, Roger Joseph

Martin, Joseph P.

Walton, John

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Geotextile behavior relevant to filtering low density suspended organic 
particles 
Marino, Roger J.; Walton, John; Cheng, S.C.; and Martin, Joseph P.  
Department of Civil, Architectural, & Environmental Engineering, Drexel University, USA 
 
 
 
Keywords:  Geotextiles, Filters, Suspended Solids, Permittivity, Water Quality 
ABSTRACT: This paper describes experiments on the feasibility of using geotextiles to filter organics from
runoff and combined sewer overflows. A horizontal flow permittivity test apparatus was used as a prototype 
to test 15cm x 20cm coupons. Non-woven, needlepunched products supported both depth (interior) and cake
(surface) filtration modes. Crushed anthracite in the #100-#200 mesh range was used as an inert, replicable 
analogue of low density, irregularly shaped suspended particles. The AOS detected with this material differs 
from that measured with standard media, indicating different interaction with the geotextile pore structure.
Drag forces maintained a cake on the upstream face of the filter even at low hydraulic gradients. The permit-
tivity decreased as this coating thickened. When flow ceased, the organic cake sloughed off the upstream face
of the coupon. Measurements made after several doses showed recovery of much of the original permittivity, 
with the loss in hydraulic capacity attributed to embedment of particles in the matrix. 
1 INTRODUCTION 
This paper examines the use of geotextile filters 
for runoff treatment.  Conventional methods such as 
sedimentation and vortexes remove settleable parti-
cles and floating debris from a discharge, but do not 
readily separate light organic constituents. Because 
microorganisms attach to these suspended and col-
loidal particles, further filtration is needed to protect 
receiving water swimmability and aquatic habitat.  
The experiments used commercially available 
geotextile products to determine the influence of 
measurable parameters (permittivity, opening size, 
porosity, etc.) on hydraulic and treatment efficiency. 
The ultimate result sought is a template for manufac-
ture of new products with internal structures opti-
mized for this new application. 
A traditional filter captures solids within its ma-
trix (“depth filter”). Eventually, it must be cleaned 
or replaced. In contrast, construction silt fences ac-
cumulate a sediment cake on the upstream face. This 
may limit seepage, but with a short service life, 
eventual clogging is acceptable. In contrast, buried 
soil filters must operate without maintenance for an 
indefinite design life. To convey seepage while 
keeping particles in place, the apparent opening size 
(AOS) and permittivity (ψ) are selected to form a 
cake upstream of the soil/filter interface. 
In removing organic particles from intermittent 
discharges at permanent outfalls, some permittivity 
loss during an event is tolerable. However, the hy-
draulic capacity must be restored by passive physical 
or biological means prior to the next storm event.  
2 BACKGROUND 
Geotextiles have been used for decades as soil fil-
ters. The intent is not filtration in the sense of cap-
ture, but to prevent solid particle migration. Using 
geosynthetics to protect landfill leachate collection 
systems is a more traditional filter application, i.e. 
interception of solids carried in the flow. Biomass 
fouling can reduce hydraulic capacity (Cancelli, A 
and Cazzuffi, D 1987), but it can stabilize with 
steady leachate permeation. The microorganisms at-
tached to filtered organic particles colonize the ma-
trix and decompose the substrate, thus re-opening 
pore channels. This sustains seepage and convection 
of substrate (Koerner, G., et al. 1994).  
Eyup Korkut (2003) showed the feasibility of us-
ing non-woven geotextiles with interior porosity to 
treat dissolved and colloidal constituents in waste-
water. Most settleable solids had already been re-
moved by sedimentation. Figure 1 is an SEM picture 
of an exhumed, air-dried filter. The attached biomass 
grew toward the outer surfaces from capture sites in 
the interior. With the biomass being a dispersed floc 
rather than a continuous film, the geotextile main-
tained a finite permittivity as in the leachate filter 
example. 
 
 
 
 
 
Figure 1. SEM picture of biomass growth in a 
nonwoven geotextile. 
 
3 PROJECT FOCUS 
This study is directed to sustainable filtration of 
harder, but still lightweight, particles suspended in 
intermittent meteorologically based flows. The “first 
flush” of urban runoff and combined sewer over-
flows (CSOs) includes pollutants that not only vary 
in nature and concentration during a storm event, but 
also with the interval since the last discharge (Pitt et 
al. 1995). Thus, the treatment performance of a Best 
Management Practice (BMP) unit is not as predict-
able as with wastewater unit operations. Most 
BMP’s require surface area and new pipe infrastruc-
ture. This restricts their application to retrofitting ur-
ban sites that have an existing pipe network, an out-
fall, and limited space and hydraulic head (Δh). 
Consequently, compact, subsurface end-of-pipe 
units are needed to separate pollutants at high ve-
locities. It is also necessary to have a provision to 
bypass a clogged filter to prevent backup into the 
drainage system. One such arrangement is described 
in (Marino et al, 2005) where filters are arrayed as 
sequential permeable baffles. Flow could clog and 
then bypass each filter in succession.  
Capture of degradable sanitary organics is en-
hanced as their “stickiness” encourages adsorption, 
but hard–surface, less degradable vegetative matter 
can also be captured by conventional types of filtra-
tion (Scheidegger, 1957) in the complex structure of 
non-woven  geotextiles. 
 With the constraint of low allowable headloss, 
the permittivity, ψ is as important as the interior po-
rosity that dominated the earlier wastewater treat-
ment study. The third index used in conventional 
geotextile soil filter design is the AOS, an indicator 
of largest through pore channel. It would appear that 
the AOS should equal the largest non-settleable sus-
pended particle size in the influent. While pore size 
distributions are not consistently related to AOS, the 
larger the AOS, the coarser the weave and the larger 
the initial permittivity and clogging resistance. 
However, the target is smaller organic particles with 
high specific surface, and attached microorganism 
density. Colloids could pass right through a coarse 
geotextile. Hence, an optimization between AOS 
(filter capture) and Q (flow treated) is in order.  
 
4 MATERIALS AND APPARATUS 
Crushed anthracite was seen as appropriate for the 
experiments: It is organic, irregular in shape, and has 
a specific gravity (1.35) similar to sanitary solids. 
Because crushed anthracite is non-degradable and 
durable, it can be used in replication tests. It was fur-
ther reasoned that if geotextile filters could intercept 
anthracite particles, they would be even more able to 
sorb degrading organic particles with a sticky gel 
coating that attracts microorganisms. Anthracite fil-
ter media from a water treatment plant was ground 
in a coffee grinder. Particle size discretization was 
obtained by mechanical sieving and wet washing.  
Commercially available non-woven nee-
dlepunched geotextiles from a single manufacturer 
were used in three mass per unit area variations. The 
manufacturer’s reported AOS values were US Sieve 
Sizes #70 for the 5 oz/sy (170 grams per square me-
ter) candidate, and the #100 sieve for both the 8.5 
oz/sy (288g/sm) and 15 oz/sy (509 g/sm) samples. 
Hence, crushed anthracite in the #100 to #200 sieve 
range was appropriate for the experiments.  
A permittivity apparatus/filter prototype was con-
structed as shown in Figure 2. It used horizontal 
flow to include gravitational separation and settling 
effects. Specifications included a large coupon size 
to compensate for variations in the fiber density, ca-
pability for low hydraulic gradients, and transpar-
ency to observe behavior. The overall dimensions of 
the Plexiglas unit are 36” (91.4 cm) long x 9” (22.9 
cm) wide x 17.5” (44.5 cm) high, with a center chan-
nel 6” wide x 8” high. Flow through the geotextile is 
governed on the upstream end by the permittivity of 
the geotextile and the applied hydraulic head. Down-
stream outflow was through two (2) 3/4” (1.91 cm) 
diameter orifices. At no point in the testing sequence 
were these orifices submerged. Therefore, flow out 
of the vessel was not restricted. 
 
 
 
 
 
Figure 2.  Geotextile Permittivity Testing Apparatus 
 
The hydraulic head was monitored with manome-
ters adjacent to the upstream and downstream sides 
of the coupon being tested. Three Δh values and 
flow rates were used in the experiments for each of 
the three geotextile densities to confirm applicability 
of Darcy’s Law as well as to vary the drag forces on 
suspended solids. For all test runs, the initial flow 
rate was determined by volume sampling over time 
(ΔV/Δt). The head differentials were standardized at 
1/8”, 1/4”, and 3/8”. These resulted in the lowest 
flow rate being 1.18 gpm for a 15 oz/sy sample for 
Δh = 1/8”, and  the highest flow rate being 10.71 
gpm for a 5 oz/sy coupon at Δh = 3/8”. Darcy’s law 
was found valid across this range. 
Average permittivity values (ψ) for each series of 
geotextile tested with clean water were computed as: 
2.23, 1.44, and 0.70 sec-1 for the 5 oz/sy, 8.5 oz/sy., 
and 15 oz/sy coupons, respectively. 
After these clean water baseline permittivity tests, 
anthracite slurry was applied in ten individual doses. 
The first dose showed that seepage drag force held 
the anthracite cake on the face of the filter. Shortly 
after flow stopped, the cake sloughed off into a 
sediment trap. Figure 3 illustrates the initiation of 
one such “peeling”. This suggested that the filters 
could rapidly reestablish some permittivity at the 
end of a storm event.  This sloughing phenomenon 
reduced the necessity of reliance on biodegradation 
to restore permittivity, and less frequent filter main-
tenance and/or replacement.  
It was decided that monitoring the behavior dose 
by dose would illustrate permittivity stability. After 
each pair of slurry doses, and the cake collapse after 
flow stopped, the residual permittivity was meas-
ured. This was repeated five times for each coupon 
until a total of ten (10) doses were applied. Figure 4 
presents results in a “sawtooth” pattern for all three 
geotextile weights tested at a hydraulic head of 1/8”. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3. Showing Sloughing After Discontinued 
 Influent Stream 
 
 
Permittivity Trend (1/8 in. Initial Head)
0.0
0.5
1.0
1.5
2.0
2.5
Ψ
 (s
ec
-1 )
5 oz 8.5 oz 15 oz
In
iti
a
D
os
e 
D
os
e 
2
S
lo
ug
h 
D
os
e 
3
D
os
e 
4
S
lo
ug
h 
D
os
e 
5
D
os
e 
6
S
lo
ug
h 
D
os
e 
D
os
e 
8
S
lo
ug
h 
 
 
Figure 4. Permittivity Trend for Anthracite Slurry Dosing At 
1/8” head 
 
The variability in measured flow and computed 
permittivity decreased as the filter mass density in-
creased, i.e., the flow and filtration performance was 
more reliable. For example, the 1/4” head resulted in 
flow rates of 7.50, 4.0, and 1.60 gpm respectively 
for the 5, 8.5, and 15 oz/sy geotextiles. The results 
of the 1/4” head permittivity test are shown on Fig-
ure 5.  
 
 
Permittivity Trend (1/4 in. Initial Head)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ψ
 (s
ec
-1 )
5 oz 8.5 oz 15 oz
In
itia
D
os
e 
D
os
e 
2
S
lo
ug
h 
D
os
e 
3
D
os
e 
4
S
lo
ug
h 
D
os
e 
5
D
os
e 
6
S
lo
ug
h 
D
os
e 
7
D
os
e 
8
S
lo
ug
h 
 
 
Figure 5. Permittivity Trend for Anthracite Slurry Dosing 
At 1/4” head 
 
With the high flow rate through the 5 oz/sy geo-
textile, backwater exceeded the height of the inlet 
reservoir when the coupon was loaded with the 
eighth dose. Therefore, hydraulic failure (surface 
clogging) had occurred through progressive increase 
in the particle mass embedded in the interior poros-
ity. Thus, there is no data reported on the following 
chart for the 5 oz/sy coupon for the fifth dose/ event. 
The mass capture (in grams) of the crushed an-
thracite is shown in Figures 6 and 7 for the 1/8” and 
1/4” head trials. It is apparent that the performance 
of the 8.5 oz/sy coupons exceeded that of the coarser 
and thinner 5 oz/sy and the finer, thicker 15 oz/sy 
samples. 
 
 
 
 
 
 
 
 
 
Capture at 1/8 in. Initial Head
1.54
3.12
2.71
2.03
4.82
2.95
0
1
2
3
4
5
6
5 oz 8.5 oz 15 oz
NW Geotextile Density
W
ei
gh
t C
ap
tu
re
d 
2 Doses
(24 g)
10 Doses
(120 g)
 
 
Figure 6. Crushed Solids Capture for Different Geotextile 
Mass per Unit Areas at 1/8” Head 
 
Capture at 1/4 in. Initial Head
1.71
3.16
2.04
2.37
4.91
4.09
0
1
2
3
4
5
6
5 oz 8.5 oz 15 oz
NW Geotextile Density
W
ei
gh
t C
ap
tu
re
d 
(
2 Doses
(24 g)
10 Doses
(120 g)
 
 
Figure 7. Crushed Solids Capture for Different  Geotextile 
Mass per Unit Areas at 1/4” Head 
 
 
Table 1 presents filtration efficiencies for the geo-
textile densities tested - where efficiency is defined 
as both the capture and the rejection of solids. Addi-
tionally, the average residual permittivity values ob-
tained for each coupon density after the sloughing 
events are shown. The data suggests that, of the 
three geotextile densities evaluated, the 8.5 oz. is the 
best candidate for capture, rejection, and residual 
permittivity. Although the 5 oz/sy coupons main-
tained a higher permittivity, they provide little inter-
nal storage and therefore, are not candidates for 
depth filtration. The 15 oz/sy coupons provide more 
internal storage, but less capture due to pore channel 
tortuosity, and also provide less permittivity than the 
8.5 oz. candidate.  
 
 
Geotextile Coupon 
Density (oz./sy) 5 8.5 15 
Average Residual 
Permittivity (sec-1) 1.2 0.7 0.4 
Average  
Capture (%) 5.4 9.9 6.5 
Average Total  
Efficiency (%) 90.4 97.1 93.7 
 
Table 1. Average Filtration Efficiencies 
for Different Density Geotextiles 
 
5 CONCLUSIONS 
Three commercially available geotextiles, having 
different mass per unit areas, were evaluated for ap-
plication as filters for storm water and combined 
sewer overflow waste streams. Non-woven needle 
punched geotextiles were selected as the filter media 
because their fibrous structure provides the opportu-
nity for both depth and cake filtration. Therefore, in-
ternal capture (in the pore channels), and external re-
jection (on the upstream face of the filter), of 
suspended solids was anticipated.  
 A permittivity apparatus/ filter prototype was 
constructed to determine both the average permit-
tivities of each fabric and the capture and rejection 
potential of the candidates. Crushed anthracite was 
used as an analogue for waste water due to its simi-
lar characteristics (specific gravity of 1.35, inert na-
ture, organic structure, amorphous shape). Several 
trials were run with the anthracite slurry through the 
prototype, and capture, rejection, filter efficiency, 
and reestablished permittivity values after sloughing 
were recorded. 
 The results suggest that geotextiles are indeed 
viable as a filtration media for the intended applica-
tion. Of the three candidates evaluated, the 8.5 oz/sy 
fabric outperformed the other two candidates in fil-
tration efficiency (the most critical parameter), cap-
ture (mass treated per flow volume), and provided 
nearly twice the permittivity of the candidate with 
the nearest filtration efficiency. 
 
 
6 REFERENCES 
Cancelli, A., and Cazzuffi, D. (1987), “Permeability 
of Geotextiles in the Presence of Water and Pol-
luted Fluids,” Proceedings of Geosynthetics 87, 
Vol.2, New Orleans, LA. 
Koerner, G.R., Koerner, R.M., and Martin, J.P. 
(1994). "Geotextile Filters Used for Leachate 
Collection Systems" J. Geotechnical Engineer-
ing, ASCE, Vol. 120.  
Korkut E.N., “Geotextiles as Biofilm Attachment 
Baffles for Wastewater Treatment”, Ph.D. The-
sis, Drexel University, 2003. 
Marino, R., Martin J. and Walton, J. (2005). “Re-
moval of Suspended Organics from Runoff with 
Geotextile Baffles” 2005 Pennsylvania Storm-
water Management Symposium, Villanova 
University, Villanova, PA USA October, 2005. 
Pitt R. E., Field, R., Lalor M., and Brown M., “Ur-
ban Stormwater Toxic pollutants: Assessment, 
Sources, and Treatability,” Water Environment 
Research, 1995 
Scheidegger, A. E. (1957) “Physics of Flow through 
Porous Media,” MacMillan Co., NY.  


Geotextile behavior relevant to filtering low density suspended organic particles

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